Contacts for reed switches



Feb. 10, 1970 s. s. COFFIN ETAL 1 CONTACTS FOR REED SWITCHES Filed July11, 1968 48 VOLTS,165 MILLIAMP LOAD O WITH ARC SUPPRESSION FAILURETYPICAL TIN -N|CKEL 10 5 1 0 2o 5o 55 5o so s5x1o INVENTORS SPENCER S.COFFIN ROBERT J. NANCE ATTORNEY United States Patent 3,495,061 CONTACTSFOR REED SWITCHES Spencer S. Collin and Robert J. Mance, Poughkeepsie,

N .Y., assignors to International Business Machines Corporation, Armonk,N.Y., a corporation of New York Filed July 11, 1968, Ser. No. 744,110Int. Cl. H01h 1/02 US. Cl. 200--166 4 Claims ABSTRACT OF THE DISCLOSURESwitch contacts consisting of a thin electroplated layer of aninter-metallic alloy of tin-nickel are provided in a reed switch of thetype where the contacts are enclosed in a sealed envelope containing aninert atmosphere. The inter-metallic alloy consists of substantially 65%tin and 35% nickel to give improved life in use under high current andarcing conditions.

The invention relates to reed switches, and more particularly, toimproved contacts for reed switches which provide a longer life underhigh current and high arcing conditions.

Reed switches and reed relays are well known in the prior art. Theyconsist essentially of thin reeds located in an inert atmosphereprovided within a sealed glass envelope. The reeds extend from oppositeends of the glass envelope and are sufficiently long that the tipsthereof overlap to form contacting surfaces. The reeds are made of aresilient magnetic material such that a magnetic field will causemovement of the reeds to close the contacts.

The contacts of reed switches or relays are generally made of a layer orlayers of materials such as rhodium, gold, silver, etc. The mainadvantage of these contact materials is that they afford a stablecontact resistence throughout their life. However, in high current highareing applications the known contact materials tend to transfermaterial from one contact to another at a particular point on thecontacts. This results in the building of a narrow pit and opposing longspire at the point which eventually causes failure of the contacts.Another disadvantage of the known contact materials is that theygenerally cannot be cheaply electroplated. Some materials, such astungsten, are very difiicult to electroplate. In the miniature reedrelay, the area over which the electroplating of a layer of contactmaterial is to be provided is exceedingly small. It is also necessary toprovide very close control of the plating operation so that a uniformthin layer can be obtained. It is necessary to maintain the layer thinso that the magnetic operation and response time of the reed switches isnot interfered with.

Accordingly, it is the main object of the present invention to provide acontact material for a reed switch which gives longer life and morereliable operation under high current and high arcing conditions.

It is another object of the present invention to provide an improvedcontact material which can be relatively cheaply electroplated in thinuniform layers.

Briefly, the invention provides improved contacts in a switch of thetype having a pair of relatively movable magnetic reed elementspositioned to overlap at a portion thereof. The overlapping portions ofthe magnetic reed are enclosed in an envelope containing an inertatmosphere. The contacts consist of a thin, uniform coating oftin-nickel applied to the facing surfaces of the overlapping portions ofthe relatively movable reed elvments. This provides a more reliablelonger life switch for use under relatively high current and arcingconditions.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of a preferred embodiment of the invention, as illustratedin the accompanying drawings.

FIG. 1 is a vertical cross sectional diagram of a reed switch.

FIG. 2 is a blown up perspective view of one of the contacts of theswitch.

FIG. 3 is a graph of the percent failure versus number of operations forswitches having tin-nickel and rhodiumon-gold contacts.

Referring to FIG. 1, there is shown a cross sectional representation ofa reed switch. The switch consists essentially of a sealed glassenvelope 12 within which is contained an inert atmosphere 14. Oneatmosphere utilized in reed switches and relays is a mixture of heliumand nitrogen i.e., helium, 20% nitrogen. A pair of reeds 16 extend intothe glass enclosure, one from each end thereof. The reeds 16 are sealedat one end in the glass 12 with their inner ends overlapping slightly.The reeds 16 are made of a magnetic material such as nickeliron. One ofthe important features of the reeds is that they be sufficientlyresilient to respond to a magnetic field. The nickel-iron reed isgenerally utilized since it has the same co-eificient of expansion asreadily available glass so that no problem arises from the sealing ofthe reed in the glass envelope. The nickel-iron makes a poor contactsurface. The overlapping contact portion 18 of the reed 16 made of thenickel-iron alloy produces a rather high contact resistance.

Another difiiculty with the use of nickel-iron is its inherent qualityof easily establishing fine bridging in switching a current from on tooff." Consequently, the high resistance and fine bridging occurs in acomparatively early state of operation and will culminate in theinability of the contacts 18 to separate. Accordingly, it has been thepractice to provide materials on the overlapping portions of the reedsto form contacts 18 which have a low contact resistance and whichminimize the fine bridging.

The present invention provides a thin, uniform layer of tin-nickel 20 onthe opposiing overlapping portions of the reeds 16. It has been found byexperimentation, that a 65% tin and 35% nickel compound orinter-metallic alloy is best suited to give a higher reliability and along life characteristic to the switch especially under high current andarcing conditions. The percentages of the materials of the compositionmay vary from the above amounts somewhat without interferring with theoperating characteristics. The thickness limitations of the layers 20are dictated by the requirements of the switch. Since the switch isoperated in response to magnetic energization, the non-magnetic materialcontact must be sufficiently thin as to not interfere with the flux pathto any significant degree. Where the magnetic characten'sics are lesscritical, the tin-nickel layer may be thicker, however, the thickness islimited by intertia characteristics which affect the response time.

For good switch operation, it is necessary to have uniform contactsurfaces. Thus, it is necessary to control the thickness of the layer ofmaterial applied to form uniform contacts 18. One way the necessaryuniformity can be obtained is thin electroplating. Various materialssuch as tungsten, rhodium, etc. cannot be easily and/or cheaplyelectroplated. This is especially true, when the small surface area overwhich the electroplating is to take place is considered. In FIG. 2, thelengthwise dimension y, which is the length of the overlapping portionsof the reed, is about 15 mils for the miniaturized reed relay.

The contacts 18 of the reed switch, in operation, transfer material fromone contact to the other. In the prior art contact materials, thetransfer from one contact to the other during operation tends to takeplace at a single point. At this single point between the contacts, apit and opposing spire or bridge tends to build up by means of which thematerial is transferred. This bridging or spire continues to grow untilthe contacts weld or are short circuited, thus, producing unreliableoperation and an early failure of the switch. In the present invention,the tin-nickel inter-metallic alloy contacts 18 transfer the materialfrom one contact to the other during operation, however, the transferdoes not appear to take place at a single point, as was the case in theprior art, but the point seems to move around the contact, thus, makinga wider pit and mound rather than a thin spire. This characteristic isbelieved to be the cause of the longer life and better reliability ofthe switch.

The tin-nickel inter-metallic alloy contacts 18 provide the aboveadvantages in operation as well as being applicable by electroplating.The process which is used to electroplate the tin-nickel layer 20 ontothe overlapping areas of the reeds 16 of the reed switches consistsgenerally of thoroughly cleaning the area to be plated. This is done byelectrochemical cleaning. After the area is thoroughly rinsed, it issubject to a chemical etch cleaning, after which it is again thoroughlyrinsed. Once the area has been thoroughly cleaned, it is immersed in anelectroplating bath. The bath used is the standard Parkinson bath forelectroplating tin-nickel as reported by N. Parkinson, TheElectrodeposition of Bright Tin-Nickel Alloy, ElectrodepositorsTechnical Society Journal, 1950-51.

In order to provide an example of the improvement obtained with thetin-nickel inter-metallic alloy contacts 18, a graph of millions ofoperations versus cumulative failure in percent for reed switches usingtin-nickel contacts and prior art rhodium-on-gold contacts is shown inFIG. 3. As can be seen from the graph, the cumulative failure inpercentage of the switches having tin-nickel contacts is per 65 millionoperations while the failure of the switches having the prior artrhodium-on-gold contacts is 40% per 10 million operations. Somewhat thesame comparatively high failure rates are obtained with various otherknown prior art contact materials when used in high energy applications.The graph was obtained using a 48 volt, 0.165 ampere inductive circuit.

Neither the tin or nickel alone provides the above advantages, but onlythe combination of the tin and nickel in an inter-metallic alloy 20forming the contacts 18 in a reed switch or relay has been found toprovide the improved reliability and long life characteristics.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and detail may bemade therein without departing from the spirit and scope of theinvention.

What is claimed is:

1. In a switch having a pair of relatively movable magnetic reedelements positioned to overlap at a portion thereof and enclosed in anenvelope containing an inert atmosphere, a thin uniform coating oftin-nickel applied to the facing surfaces of said overlapping portionsof said relatively movable reed elements forming contacts thereon,whereby the life and reliability of the switch is increased in use underrelatively high current and arcing conditions.

2. A switch according to claim 1, wherein said tinnickel contactscontain approximately tin and 35% nickel.

3. A switch according to claim 1, wherein said tinnickel coatingthickness is primarily limited by the magnetic operating characteristicsof said switch.

4. A switch according to claim 1, wherein said tinnickel contacts areapplied by an electroplating process to obtain the uniform desiredthickness.

References Cited UNITED STATES PATENTS 2,882,648 4/1959- Hovgaard et al.

3,007,855 11/1961 Ellwood.

3,222,486 12/1965 Moriyama et al.

3,251,121 5/1966 Prival.

3,426,302 2/1969 Lindenberger 29622 X 3,431,377 3/1969 Barlow et al.

HERMAN O. JONES, Primary Examiner U.S. Cl. X.R. 29622

